1. Field of the Invention
The present invention relates to a wafer flattening system for locally etching projections on a wafer surface by an activated species gas to flatten the surface or locally etching relatively thick portions of a wafer to achieve a uniform distribution of thickness of the wafer.
2. Description of the Related Art
FIG. 25 is a schematic sectional view of an example of a wafer flattening technique of the related art.
In FIG. 25, reference numeral 100 is a plasma generator. Activated species gas G in the plasma generated by the plasma generator 100 is sprayed on the surface of a wafer W from a nozzle 101.
The wafer W is placed and secured on a stage 120. The stage 120 is made to move in the horizontal direction to guide a portion Wa relatively thicker than a prescribed thickness on the surface of the silicon wafer W (hereinafter referred to as a xe2x80x9crelatively thick portionxe2x80x9d) directly under the nozzle 101.
The activated species gas G is then sprayed from the nozzle 101 to the projecting relatively thick portion Wa to locally etch the relatively thick portion Wa to achieve a uniform distribution of thickness of the surface of the wafer W and thereby flatten the surface of the wafer W.
The above wafer flattening technique of the related art, however, suffered from the following problem.
The ions in the plasma generated by the plasma generator 100 are accelerated by the potential difference applied between the plasma and wafer W in striking the wafer W. Only the portions struck by the ions are etched by large extent. Further, due to sputtering, the atoms of the surface of the wafer W are stripped away. Therefore, the surface of the wafer W becomes rough at the atomic order.
Further, particles floating around the wafer W and particles generated in the discharge tube forming the nozzle 101 deposit on the surface of the wafer W causing the etchability of the portions where the particles are deposited to decline. As a result, the amounts of etching at the portions where the particles are deposited and the portions where they are not deposited differ and the surface of the wafer W is again roughened.
Due to the above, when local etching is performed, the root mean square roughness (RMS) of the surface of the wafer W ends up becoming larger. When the surface of the wafer W was actually observed after local etching under an interatomic microscope, it was learned that when a wafer W having an RMS before local etching of less than 1 nm is locally etched by the above wafer flattening process, the RMS ends up deteriorating to as bad as about 10 nm.
Further, when transferring the wafer W from a previous step to the local etching step, an oxide film naturally forms on the surface of the wafer W. If the wafer is locally etched in a state with this natural oxide film left alone, white turbidity will form at the surface of the wafer W after the local etching and will cause a deterioration of the surface roughness of the wafer W.
The present invention was made to solve the above problems and has as its object to provide a wafer flattening system configured to continuously and automatically perform the removal of the natural oxide film of a wafer and the flattening and smoothing of the wafer so as to improve the surface roughness of the wafer and to increase the work efficiency.
To achieve the above object, according to the aspect of the invention, there is provided a wafer flattening system comprising: a natural oxide film removing device for removing a natural oxide film formed on a wafer surface; a first transport device for taking out a wafer from which the natural oxide film has been removed by the natural oxide film removing device and transporting it to a predetermined location; a local etching apparatus provided with a discharge tube with a nozzle facing the wafer transported by the first transport device and a plasma generator for causing plasma discharge of a gas of a fluorine compound or a mixed gas including a fluorine compound fed to the discharge tube so as to produce a predetermined activated species gas and spraying the activated species gas from the nozzle of the discharge tube to a relatively thick portion of the surface of the wafer to locally etch the relatively thick portion; a second transport device for taking out a wafer flattened by the local etching apparatus and transporting it to a predetermined location; and a smoothing apparatus for grinding off a projection on the surface of the wafer transported by the second transport device to smooth the wafer surface.
Due to this configuration, the natural oxide film formed on the wafer surface is removed by the natural oxide film removing device, then the wafer is transported by the first transport device to the local etching apparatus. Further, at the local etching apparatus, the plasma generator causes plasma discharge of the gas of the fluorine compound or the mixed gas including the fluorine compound fed into the discharge tube so as to produce the activated species gas. This being done, the activated species gas is sprayed from the nozzle of the discharge tube to relatively thick portions of the wafer, whereby the relatively thick portion is locally etched. Therefore, by successively locally etching the entire surface of the wafer, the entire wafer surface is flattened. Further, the flattened wafer is transported by the second transport device to the smoothing apparatus. The smoothing apparatus grinds off the projections on the surface of the wafer to smooth the wafer surface.
In particular, according to an embodiment of the invention, the first transport device and the second transport device are configured by a single transport device used for both purposes.
As an example of the natural oxide film removing device, according to an embodiment of the invention, the natural oxide film removing device is provided with a solution tank for storing a solution for removal of the natural oxide film of the wafer, a washer, and a third transport device for immersing the wafer in the solution tank and then transporting it to the washer.
Due to this configuration, the wafer is immersed by the third transport device in the solution tank storing the solution for removal of the natural oxide film. After the removal of the natural oxide film, the third transport device is used to transport the wafer to the washer where the solution and other unnecessary matter are washed off.
There are various types of solutions for removal of the natural oxide film, but as preferable examples, according to an embodiment of the invention, the solution for removal of the natural oxide film stored in the solution tank is an aqueous solution of hydrofluoric acid.
As another example of the natural oxide film removing device, according to an embodiment of the invention, the natural oxide film removing device is provided with a discharge tube for spraying a predetermined activated species gas over the entire surface of the wafer and a plasma generator for causing plasma discharge of a mixed gas including a fluorine compound and hydrogen in the discharge tube so as to produce the activated species gas.
Due to this configuration, the plasma generator causes plasma discharge of the mixed gas containing the fluorine compound and hydrogen in the discharge tube and the production of the activated species gas. This being done, the activated species gas is sprayed from the discharge tube over the entire surface of the wafer, whereby the natural oxide film on the wafer surface is etched by the activated species gas.
Note that as a preferable example of the ratio of mixture of the mixed gas, according to an embodiment of the invention, the ratio of the hydrogen to the fluorine compound in the mixed gas is from 0.1% to 50% by volume.
Further, it is preferable to position a flat or notch serving as a mark for identification of the crystallization direction of the wafer or wafer position at the time of the local etching processing. Therefore, according to an embodiment of the invention, the natural oxide film removing device is provided with a positioning mechanism for positioning a flat or notch of the wafer.
As an example of a smoothing apparatus, according to an embodiment of the invention, the smoothing apparatus is a chemical mechanical polishing apparatus provided with a platen having a polishing pad on its surface and being capable of rotating, a carrier for holding the surface of the wafer in a state facing the platen and rotating while pressing the wafer against the polishing pad of the platen, and a polishing slurry feeder for feeding a predetermined polishing slurry between the wafer surface and the polishing pad.
The activated species gas G2 produced by this plasma discharge is sprayed from the nozzle 61a of the alumina discharge tube 61 and diffuses over the entire surface of the silicon wafer W. Due to this, the surface of the silicon wafer W is etched by the F radicals or O radicals contained in the activated species gas G2. That is, when the activated species gas G2 is sprayed on to the surface of the silicon wafer W, as shown in FIG. 16, due to the presence of O radicals, a reaction product R thought to be SiOxFy (x, y=1, 2 . . . ) is produced. Further, the F radicals etch the surface of the silicon wafer W. Further, the reaction product R is produced on the entire surface of the silicon wafer W, but due to the vapor pressure, the majority of the reaction product R evaporates, while the reaction product R is deposited in the slight depressions Wc causing roughness to remain without evaporating. Therefore, the reaction product R deposits in the depressions Wc to protect the depressions from etching by the F radicals, while only the portions other than the depressions Wc are etched by the F radicals.
Due to this configuration, the wafer is held by the carrier in a state with its surface facing the platen side and the wafer is pressed against the polishing pad of the rotating platen while rotating. Further, the polishing slurry feeder feeds the polishing slurry between the wafer surface and the polishing pad, whereby the wafer surface is smoothed to a mirror finish surface.
As another example of a smoothing apparatus, according to an embodiment of the invention, the smoothing apparatus is provided with a discharge tube for spraying a predetermined activated species gas over the entire surface of the wafer and a plasma generator for causing plasma discharge of a mixed gas of a fluorine compound and oxygen in the discharge tube to produce the activated species gas.
Due to this configuration, the plasma generator causes plasma discharge of the mixed gas containing the fluorine compound and oxygen in the discharge tube and the production of the activated species gas. This being done, the activated species gas is sprayed from the discharge tube over the entire surface of the wafer, whereby the entire surface of the wafer is etched and smoothed by the activated species gas.
Further, there are various types of fluorine compounds. As examples, according to an embodiment of the invention, the fluorine compound in the mixed gas is selected from the group consisting of carbon tetrafluoride, sulfur hexafluoride, and nitrogen trifluoride.
Further, as a preferable example of the ratio of mixture of the mixed gas, according to an embodiment of the invention, the ratio of the oxygen to the carbon tetrafluoride in the mixed gas is 200% to 400% by volume.
Further, it is possible to provide a plurality of local etching apparatuses. Therefore, according to an embodiment of the invention, there is provided a wafer flattening system further comprising: N (an integer of 2 or more) number of local etching apparatuses; and a controlling apparatus provided with a natural oxide film removing device controller for controlling the first transport device to load an unprocessed wafer to a not operating natural oxide film removing device, making the natural oxide film removing device operate for exactly a time enabling natural oxide film removal, then controlling the first transport device to transport the wafer finished being removed of the natural oxide film from the natural oxide film removing device to a not operating X-th (integer from 2 to N) local etching apparatus, a local etching apparatus controller for making the X-th local etching apparatus operate for exactly a time enabling local etching, then controlling the second transport device to transport the wafer finished being locally etched from the local etching apparatus to the not operating smoothing apparatus, and a smoothing apparatus controller for making the smoothing apparatus operate for exactly a time enabling smoothing, then controlling the second transport device to unload the wafer finished being smoothed from the smoothing apparatus.
Due to this configuration, the first transport device loads the unprocessed wafer to a not operating natural oxide film removing device under the control of the natural oxide film removing device controller of the controlling apparatus. Further, the natural oxide film removing device is operated for exactly the time enabling natural oxide film removal under the control of the natural oxide film removing device controller, then the first transport device is made to transport the wafer finished being removed of the natural oxide film from the natural oxide film removing device to a not operating X-th local etching apparatus. Further, the X-th local etching apparatus is operated for exactly the time enabling local etching under the control of the local etching apparatus controller, then the second transport device is made to transport the wafer finished being locally etched from the local etching apparatus to the not operating smoothing apparatus. Suitably thereafter the wafer smoothed at the smoothing apparatus is unloaded under the control of the smoothing apparatus controller. That is, a plurality of wafers after processing for natural oxide film removal are transported to a plurality of not operating local etching apparatuses. The plurality of local etching apparatuses enable the plurality of wafers to be locally etched at one time. Further, the wafers are transported to the smoothing apparatus in order starting from the first wafer finished being locally etched.
In particular, according to an embodiment of the invention, a transport sequence controller for controlling the natural oxide film removing device controller is provided at the controlling apparatus so that the first transport device transports a wafer finished being removed of the natural oxide film to the local etching apparatus with the longest non-operating time among the local etching apparatuses finishing local etching processing.
Due to this configuration, when transporting a wafer finished being removed of the natural oxide film to a not operating local etching apparatus, if there are a plurality of not operating local etching apparatuses, the wafer is transported to the local etching apparatus with the longest non-operating time after stopping operation.
Further, according to an embodiment of the invention, N greater than m is set when the time enabling local etching in a local etching apparatus is m (positive integer) times the time enabling natural oxide film removal in the natural oxide film removing device.
Further, if it were possible to have the natural oxide film removing step, the local etching step, and the smoothing step performed by a single apparatus, it would be possible to save the time for transport of the wafers to the different steps and possible to make the processing work further efficient. Therefore, according to an aspect of the invention, there is provided a wafer flattening system comprising: an etching apparatus provided with a discharge tube with a nozzle facing a surface of a wafer and a plasma generator for causing plasma generation of a predetermined gas fed to the discharge tube so as to produce a predetermined activated species gas and spraying the activated species gas from the nozzle of the discharge tube to the surface of the wafer to etch the wafer surface; a gas feeder capable of feeding a gas into the discharge tube; a transfer mechanism for shortening or increasing the distance between the nozzle of the discharge tube and the wafer; and a first automatic controller provided with a natural oxide film removing processing controller for controlling the transfer mechanism to increase the distance between the nozzle and the wafer until the activated species gas sprayed from the nozzle will diffuse over the entire surface of the wafer and controlling the gas feeder to feed a mixed gas containing a fluorine compound and hydrogen to the inside of the discharge tube, then driving the etching apparatus, a local etching processing controller for controlling the transfer mechanism to shorten the distance between the nozzle and the wafer until the activated species gas sprayed from the nozzle will locally strike a relatively thick portion of the wafer and controlling the gas feeder to make it feed a gas of a fluorine compound or a mixed gas including a fluorine compound into the discharge tube, then driving the etching apparatus, and a smoothing processing controller for controlling the transfer mechanism to increase the distance between the nozzle and the wafer until the activated species gas sprayed from the nozzle will diffuse over the entire surface of the wafer and controlling the gas feeder to make it feed a mixed gas containing a fluorine compound and oxygen to the inside of the discharge tube, then driving the etching apparatus.
Further, according to an embodiment of the invention, there is provided a wafer flattening system further comprising: N (an integer of 2 or more) number of etching apparatuses; a loader for loading a wafer into the etching apparatus; an unloader for unloading a wafer from the etching apparatus; and a second controller for driving the loader to make it load N number of unprocessed wafers inside the N number of etching apparatuses, then driving the first automatic controller and then, after the elapse of a predetermined time, driving the unloader to unload the N number of processed wafers from the N number of etching apparatuses.